Microscale Simulation of Carbon Support Structure Degradation in Polymer Electrolyte Fuel Cell

Abstract

Understanding the mechanism of Polymer electrolyte fuel cell (PEFC) degradation plays an important role in improving fuel cell performance and durability. For the PEFC system, the structure of catalyst layers (CLs) is the critical component that determines cell performance. The PEFC degradation due to the reduction of carbon on the CLs has been investigated in previous studies [1]. In our past studies, we focused on the actual heterogeneous porous structure of catalyst layer. From these results, it was found that mass transport performance of oxygen and proton in heterogeneous porous catalyst layer, which consists of carbon support, ionomer and void space, is much lower than that of theoretical prediction in homogeneous porous media [2-5]. In addition, it depends on the structure of carbon black, the coating structure of ionomer and the local distribution of these materials. Thus, it is important to understand and to predict the carbon support structure change by degradation because this structural degradation strongly affect the mass transport performance. In this work, the effect of CLs porous carbon support degradation on cell performance is investigated by microscale simulation.The CLs structures, consisting of porous carbon, ionomer, and Pt, are created by our previous study multi-block method [2,6]. In our simulation, we focus on studying the effect of CLs' beginning-of-life (BOL) and end-of-life (EOL) structure in the degradation process. The CLs creation procedure is illustrated in Fig. 1. Firstly, CLs structure are created by packing CB and covered ionomer and Pt. The amount of ionomer and Pt are assumed as constant during the degradation process. Therefore, the amount of carbon is reduced in EOL structure. In this study, various CLs structures with decreasing of carbon volume are constructed. Furthermore, the mechanical stress distribution under compression condition was simulated in each structure to understand the effect of framework of carbon on local porosity in catalyst layer. The effect of carbon corrosion in CLs is investigated by calculating the cell performance of PEFC of each CLs structure by ORR mass transport and electrochemical reaction calculation with our previous method [6].The cell performance is significantly decreased by reducing carbon. The degradation mechanism of PFEC is explained by degradation of not only effective surface area but also the porous structure change which affect oxygen diffusion performance. Moreover, compared with BOL structure the apparant thickness of covered ionomer layer is higher in EOL structures because of carbon corrosion and constancy of ionomer. Hence, the higher oxygen diffusion resistance in secondary pore and ionomer layer occur in CLs degradation structures. The effect of this dynamic structure change on cell output performance will be reported in this presentation.Acknowledgements:This research was supported by the New Energy and Industrial Technology Development Organization (NEDO), Japan, grant number P20003-20001327-0.